The present embodiments relate to wireless communication systems and, more particularly, to Coordinated Multi-Point (CoMP) transmission in which a single mobile unit communicates with plural base stations.
In legacy wireless cellular systems such as LTE Rel. 8 to 10, a wireless network includes multiple base stations. Each base station may be configured as a single cell with their own cell ID. A mobile terminal or user equipment (UE) is always synchronized to and communicates Uplink (UL) and Downlink (DL) data with the connected cell with single-cell transmission/reception.
Channel station information reference symbol (CSI-RS) is used in LTE Rel. 10 for UE CSI feedback purpose. A UE measures the downlink channel from an eNB transmitter to the UE receiver using CSI-RS and reports Channel State Information (CSI) measurement in the uplink. CSI-RS is UE-specific and unprecoded. CSI-RS configuration in LTE Rel. 10 is based on the single-cell framework. The following parameters for CSI-RS are explicitly configured via semi-static radio resource control (RRC) higher-layer signaling for each UE, including the following parameters Nt, Ni, Np, Noffset and α. Nt is the number of CSI-RS antenna ports. In LTE Rel. 10 the number of antenna ports can be ′, 2, 4 or 8. Ni is the CSI-RS pattern index corresponding to a certain CSI-RS pattern, based on the number of CSI-RS antenna ports. Np is the duty cycle or periodicity of CSI-RS transmission. For Np=5 the CSI-RS is transmitted every 5 subframes. In LTE each subframe is 1 ms in duration. Noffset is the subframe offset. The duty cycle and subframe offset are jointly encoded in LTE Rel. 10 and signaled to a UE via the downlink subframes that contain CSI-RS. The parameter α is used to control UE assumption on reference PDSCH transmitted power for CSI feedback.
Tables 1 and 2 below list examples of these quantities.
TABLE 1ParametersValuesNumber of CSI-RS2, 4, 8Antenna PortsIntra-subframe32/16/8 patterns forlocation index2/4/8 CSI-RS ports
TABLE 2Icycle, subframe offsetDuty cycleSubframe offset   Icycle, subframe offset ≦ 45Icycle, subframe offset   5 ≦ Icycle, subframe offset ≦ 1410Icycle, subframe offset − 5 15 ≦ Icycle, subframe offset ≦ 34 20Icycle, subframe offset − 1535 ≦ Icycle, subframe offset ≦ 74 40Icycle, subframe offset − 3574 ≦ Icycle, subframe offset ≦ 15480Icycle, subframe offset − 75
The CSI-RS sequence mapped to each CSI-RS pattern in a cell is generated by a pseudo-random sequence generator as a function of the cell ID in the cell.
In Rel. 10 the cell ID is not explicitly signaled by the eNB but is implicitly derived by the UE as a function of the primary synchronization signal (PSS) and secondary synchronization signal (SSS). To connect to a wireless network, the UE performs downlink cell search to synchronize to the strongest cell. Cell search is performed by blindly detecting the PSS/SSS of each cell and comparing the receive power strength of different cells. After cell search is successfully performed, the UE establishes connection to the strongest cell and derives the cell ID from the PSS/SSS.